THE recent work of Hickling and Salt1 in proposing a new version of the atomic hydrogen theory of overpotential has served to increase interest in experiments which offer critical evidence differentiating between the theories concerning the various processes regarded as the slow stage in overpotential. Little work has been done on the influence of the solvent on overpotential2, and knowledge of this latter aspect would seem of use in the connexion mentioned above. Thus, alteration of the solvent medium at once affects the entities discharged at the cathode, the interfacial tensions at the metal–solution and solution–gas interfaces and adsorption on the cathode. Each of these factors has been regarded as having considerable importance in hydrogen overpotential theory3.